Casting Metal Parts
Recently I tried casting parts in tin based metal. This is appealing to me for several reasons. The primary being that Brittania Metal is for less toxic as compared to the resins used for casting. I use a metal that is made up of 89% Tin (Sn derived from the latin Stannum) 8% Antimony (Sb from the latin stibium) and the remainder is Copper (Cu or cuprum). Combined they create a easily cast metal that is non toxic and will cast well at temperatures under 575 fahrenheit. The lower melting temperatures still allow us to use RTV rubber for the mold material. I use Smooth-On Mold Max-60 High Temperature Tin Cured Silicone Rubber.
(Mold Max® 60 is a Shore 60A tin catalyzed silicone rubber formulated for applications requiring high heat resistance (up to 560°F/294°C). It features a low mixed viscosity and cured rubber exhibits very low linear shrinkage. Parts A & B are mixed 100A & 3B by weight. Pot life is 40 minutes and cure time is 24 hours. Applications include making foundry patterns, flat pattern reproductions and casting low-melt metal alloys such as tin and pewter.)
Lets get started. You will need a master that you want to reproduce first. I will use the deck for my Pechot Bogie project as my subject for this tutorial. The deck has detail on all sides and to complicate matters it has very fine detail on both top and bottom. See Images below.
Top Detail:
Bottom Detail:
In most cases I take the side with the least amount of detail and push the master into the clay base, in this case either side would be too fragile to push into the clay. To work around this I cut a hole in the plexi to exactly surround the bottom detail. I could then lay a thin film of clay around the perimeter of the hole creating a seal and not run the risk of damaging the master before the rubber has been poured. I can now lightly press the master into the clay without damaging the under detail. Once the master is positioned I make sure the clay has sealed around the master edges using a round tipped tool. This will prevent any rubber from leaking out of the mold box. Next I will form the pour spouts or channels. I try to make these as large as possible to create extra force using gravity when pouring the metal into the mold. The sprue metal can be reused so it is not wasting material.
TIP: Always make sure you are not creating any negative cavities that will keep the part or casting locked into the mold. The RTV rubber used for higher temperature casting is not as flexible as the softer rubbers used with resin and the part will lock into the mold with any negative cavity. The rubber also tears easily so parts must be angled in such a way that they would come out easily.
The image series below shows the mold placement clay sequence. I also use a 0.25" round tip tool to create indents that will serve as registration points, as they will be mirrored on the second mold half.
Master aligned with base to make sure bottom detail fits in cavity.
Thin layer of clay in which the master will be lightly pressed. * The clay must be Sulphur Free Clay*
Master pressed into clay and pour vents added.
